1. Field of the Invention
The present invention relates to a single acting hydraulic or pneumatic actuator ensuring, by means of a pneumatic energy accumulator, the return thereof to its initial position and holding same in this position when the pressure of the hydraulic drive fluid drops below a predetermined pressure. This initial position is generally a safety position for the controlled device.
It applies more particularly, but not exclusively, to the rotational drive of the operating shaft of a device such as a valve or a cock.
2. Description of the Prior Art
For this type of application numerous embodiments have already been proposed.
Thus, for example, from French Pat. No. 1 460 561, an actuator is known using a hydraulic or pneumatic cylinder whose rod is connected to the drive shaft of the cock through a transmission system irreversibly converting the rectilinear movement of the rod of the cylinder into a quarter of a turn rotational movement of the shaft driving the valving member of the cock. In this embodiment, the cylinder is double acting and controls in one direction the opening and, in the other direction, the closing of the cock. However, a disadvantage of this type of actuator consists in requiring, both for opening and closing, the admission into the cylinder of a pressurized drive fluid, so that in the absence of pressurized drive fluid, for example following a breakdown of the pressurized fluid generator or a break in the ducts supplying the cylinder, the actuator remains in the position which it occupied at the time of the breakdown. Thus, if the cock is open, it will remain open without the immediate possibility of being closed again (or conversely).
To overcome these drawbacks, actuators have also been proposed of the kind using a single acting cylinder for operating the valving member of the cock and an energy accumulation device for returning the valving member of the cock to the safety position and holding it there when the pressure of the drive fluid drops below a certain value. The energy accumulation devices used in these actuators often comprise springs, for example simple helical springs, which are compressed by the rod of the cylinder when this latter is subjected to the pressure of the drive fluid and absorb, in the form of potential energy, a part of the energy of the cylinder and which restore this energy by pushing the rod of the cylinder back to its initial position when this latter is no longer driving. French Pat. No. 2 304 957 in particular describes an energy accumulation system for obtaining, during the return of the actuator to its initial position and in the absence of control pressure, a return force whose value varies as a function of the movement of the rod of the cylinder according to a law appropriate for the desired aims.
However, these energy accumulation actuators, having a mechanical structure, present a number of drawbacks:
They are generally heavy and relatively cumbersome and sometimes complex from the mechanical point of view, which adversely affects their reliability.
They require the use of relatively powerful actuating cylinders with respect to the force required for providing only operation of the cock (opening or closing). In fact, during operation of the cock, the cylinder must produce a double force (the force required for effectively opening the cock plus the force applied to the accumulation device for storing the energy required for returning the cock to the safety position).
They are subject to wear, corrosion and/or spring fatigue phenomena so that in the long run the safety function may become defective.
The aim of the invention is then to overcome all these disadvantages. For this it provides an actuator in which the energy accumulator is of the pneumatic type and in which the charge of the accumulator with potential energy is provided by the pressurized drive fluid which supplies the cylinder.